1. Field of the Invention
The present invention relates to an apparatus for forming an electrostatic latent image on a surface of a photoreceptor by using a semiconductor laser beam modulated by an image signal, and more particularly to an electrostatic latent image forming apparatus of a type in which bias current is added to current for exciting a semiconductor laser.
2. Description of the Prior Art
A so-called electrographic process in which an electrostatic latent image formed on a photoreceptor is developed by toner and transferred on paper is conventionally utilized as a printing technique in a copying machine, a printer or the like. As an apparatus for forming such an electrostatic latent image, there is known an apparatus in which a laser beam modulated by an image signal obtained by exposure and scanning of an image of a document is applied to a photoreceptor to form an electrostatic latent image on the surface of the photoreceptor. According to a proposal in the prior art, such an apparatus comprises a temperature compensating circuit.
FIG. 1 is a graph showing optical output and forward current characteristics (referred to hereinafter as P-I characteristics) of a semiconductor laser used for such an electrostatic latent image forming apparatus. In FIG. 1, the abscissa represents forward current (I) and the ordinate represents optical output (P).
As can be seen from FIG. 1, the P-I characteristics of the semiconductor laser show a sharp rise with respect to a certain current value as a boundary value which is varied according to the ambient temperature. In other words, the P-I characteristics of the semiconductor laser show parallel movements in the direction of the axis I according to the change of the ambient temperature. Thus, the optical output of the semiconductor laser changes largely dependent on the change of the temperature, causing a disturbance to image data.
FIG. 2 is a block diagram showing an automatic power control (APC) circuit proposed for the purpose of compensating for such change of optical output due to temperature change. Such an APC circuit is disclosed in detail for example in Japanese Laid-Open Patent application No. 170280/1985.
First, configuration of the APC circuit in FIG. 2 will be described.
Referring to FIG. 2, a switching portion 1 generates signal current or switching current Isw for modulating a laser beam in response to an image signal applied from a microcomputer M/C (not shown) so that the signal current or the switching current Isw is applied to a semiconductor laser L.D. An optical output level of the semiconductor laser L.D driven by the signal current is detected by a photodiode P.D and fed back to an APC portion 2. The APC portion 2 generates bias current Iba and adds it to the signal current Isw so that the optical output level of the semiconductor laser L.D may be constant. More specifically, the semiconductor laser L.D is driven by current Iop obtained by adding the bias current Iba to the signal current Isw.
Referring to FIG. 3, the operation principle of the APC circuit shown in FIG. 2 will be described. The upper half portion of FIG. 3 is a graph showing P-I characteristics of the semiconductor laser in the same manner as in FIG. 1 where the abscissa represents forward current (Iop) and the ordinate represents optical output (P). In FIG. 3, the optical output level Ps is a level corresponding to sensitivity of a photoreceptor, where an electrostatic latent image can be formed. More specifically, if a laser beam of an output level higher than the level Ps is applied to the photoreceptor, the potential at the incident point on the surface of the photoreceptor is sufficient to permit development by the toner (adhesion of the toner). On the contrary, if a laser beam of an output level lower than the level Ps is applied to the photoreceptor, the potential at the incident point on the photoreceptor is not sufficient for adhesion of the toner.
In this case, as shown in FIG. 3, the optical output level P1 (P1&gt;Ps) is adopted as an "on-level" signal of the optical output, which means an optical output level causing the potential of the incident point of the laser beam on the photoreceptor to be lowered to a level where the toner can be adhered.
In FIG. 3, if a case of the ambient temperature at 10.degree. C. is considered, a current value I10 necessary for obtaining the above mentioned on-level optical output P1 is as follows. EQU I10=Isw+Iba (1)
Similarly, a current value I25 necessary for obtaining P1 at temperature 25.degree. C. is as follows. EQU I25=Isw+Iba' (2)
In addition, a current value I50 necessary for obtaining P1 at temperature 50.degree. C. is as follows. EQU I50=Isw+Iba" (3)
In other words, current of Iba at temperature 10.degree. C., current of Iba' at temperature 25.degree. C. and current of Iba" at temperature 50.degree. C. are respectively added, as the bias current, to the switching current Isw so that the constant on-level optical output P1 can be obtained irrespective of the change in the temperature.
As a conventional electrostatic latent image forming apparatus, there is known an apparatus in which a beam attenuation filter is disposed between a laser beam generator and a photoreceptor. Such an apparatus is disclosed for example in Japanese Laid-Open Patent application No. 23914/1982 or 146017/1984. Such a beam attenuation filter is provided for the below described reason. A conventional apparatus generally comprises an SOS (start-of-scan) sensor for defining a printing start position and it is necessary to apply sufficient intensity of light to this sensor for rapid response. However, if a too intense beam is applied to the photoreceptor having high sensitivity, there are caused unfavorable effects such as deterioration of the sensitivity or damage of the photoreceptor. Therefore, a beam attenuation filter is disposed in a light path between the laser beam generator and the photoreceptor except for a light path for the SOS sensor so as to solve the above described incompatible problem. However, it is to be noted that those known techniques make no disclosure as to application of bias current to signal current for the purpose of assuring a constant optical output level as described above.
Returning again to the circuit in FIG. 2, if the switching current is cut off, namely, Isw=0, the optical output level PO is as follows. EQU PO.noteq.0 (4)
More specifically, even if the switching current is cut off (Isw=0), the optical output level PO is not completely equal to 0 because of the above described bias current (Iba at 10.degree. C., Iba' at 25.degree. C. and Iba" at 50.degree. C.) and a weak optical output is generated.
Such a weak optical output PO is as follows. EQU PO&lt;Ps (5)
Thus, the optical output PO is in an "off-level" region, which means a region of an optical output level where the potential of the incident point of the laser beam on the photoreceptor is not sufficient to a level enabling adhesion of the toner.
However, there is a probability of adhering the toner at a boundary between the on-level region and the off-level region and even in the off-level region, although this probability is of a low degree according to the optical output of the bias current.
As a result, by adhesion of the toner occurring with such a low probability, a small amount of the toner uniformly adheres even in a range which should be white on a reproduced image, causing a so-called "fogging" phenomenon.
In addition, if a copying machine is in a wait state or in a state of initialization or the like, the above mentioned weak optical output continues to be applied to a specified region of the photoreceptor, resulting in optical fatigue.
Furthermore, such application of bias current is effected not only for the above described purpose of temperature compensation but also for the purpose of improving switching characteristics of the semiconductor laser and similar problems also occur for the latter purpose.